Disk files are information storage devices that use a rotatable disk with concentric data tracks containing the information, a head for reading data from or writing data onto the various tracks, and an actuator connected by a support arm assembly to the head for moving the head to the desired track and maintaining it over the track centerline during read or write operations. The movement of the head to a desired track is referred to as track accessing or "seeking", while the maintaining of the head over the centerline of the desired track during a read or write operation is referred to as track "following".
The actuator is typically a "voice coil motor" (VCM), which comprises a coil movable through the magnetic field of a permanent magnetic stator. The application of current to the VCM causes the coil, and thus the attached head, to move radially with respect to the disk. The acceleration of the coil is proportional to the applied current, so that ideally there is no current to the coil if the head is perfectly stationary over a desired track.
In disk files having a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to maintain the head precisely over the centerline of the desired track during read or write operations. This is accomplished by using prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. The servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a position error signal (PES), which is an indication of the position error of the head away from the nearest track centerline. One type of conventional servo pattern for use with either a sector servo disk or a dedicated servo disk is the quadrature pattern described by Herrington and Mueller in IBM TECHNICAL DISCLOSURE BULLETIN, Vol. 21, No. 2 (July 1978) pp. 804-805. In the quadrature pattern there are four unique track types which repeat so as to form radially repetitive four-track bands of servo information.
During track seeking, when the head is moving across the tracks, the PES is used to generate track crossing pulses. This track crossing information, together with the PES and a signal representing the desired or target track, is used to generate a total error signal. The total error signal is equal to the sum of the PES plus the difference between the position of the target track and the position of the actual track over which the head is located. The total error signal is then used in the servo feedback loop to compute the desired velocity of the head, via a reference velocity trajectory generator, to assure that the head arrives at the target track according to the optimum velocity trajectory to effect a move to the target track in minimum time. The computed velocity is then compared with the estimated velocity from an electronic tachometer to generate a velocity error signal to a power amplifier which in turn applies a control current to the VCM. An electronic tachometer for generating a velocity estimate from inputs of VCM control current and quadrature PES is described in U.S. Pat. No. 4,246,536 to Bradley, et al., which is assigned to the same assignee as this application.
During track following, when the head is located within the boundaries of the desired track, the PES alone is used in the servo feedback loop to generate a control signal to the VCM to move the head back to the track centerline.
A description of operation of a general disk file servo control system during track seeking and track following is given by R. K. Oswald in "Design of a Disk File Head-Positioning Servo", IBM JOURNAL OF RESEARCH AND DEVELOPMENT, November 1974, pp. 506-512.
In such conventional disk files, the use of track crossing pulses to determine the total error signal during a track seek requires additional costly and complex analog circuitry in the demodulator to generate the track crossing pulses from the PES. More importantly, in the case of disk files which use sector servo data, it is not possible to accurately count track crossing pulses directly from the PES since the head will typically have crossed numerous tracks between PES samples.
In the past, many computer hard disk files have been equipped with analog servo systems for controlling the movement of the head and arm radially over the disk surface. More recent products however incorporate a digital servo mechanism.
A digital servo control system is described in U.S. Pat. No. 4,679,103 to Workman. As described in that patent, the digital servo control system receives as input, at discrete sample times, the position error signal and a digital value representation of the voice coil motor current.
Included in the digital servo control system is a digital processor that controls the overall servo performance. Typically, position error signal (PES) circuits in the head and arm signal channel are analog, and represent the position of the head on the disk. For digital processing, the analog position error signal is converted to a digital signal using a conventional analog-to-digital converter (ADC). The digital PES signal may then be read by the digital processor.
A simplified block diagram of a digital servo control system is shown in FIG. 1. Such a digital servo control system is described in more detail in U.S. Pat. No. 4,679,103 to Michael C. Workman, entitled "Digital Servo Control System for a Data Recording Disk File," which is assigned to the same assignee as this application.
A pair of disks 10, 12 are supported on a spindle 14 of the disk file drive motor 16. Each of the disks 10, 12 has two surfaces 20, 22 and 24, 26, respectively. For purposes of this description, the surface 20 on the disk 10 and the surfaces 24, 26 on the disk 12 are data recording surfaces. The surface 22 on the disk 10 is a dedicated servo surface and contains only prerecorded servo information. The servo information is recorded in concentric tracks and is typically written in such a manner that the intersections of adjacent servo tracks on the servo surface 22 are radially aligned with the centerlines of the data tracks on the surfaces 20, 24, and 26. The servo information on the surface 20 may be the quadrature pattern, as described in the previously cited reference by Mueller, et al.
The specific tracks on the data disks and the servo disk are accessed by heads 30, 32, 34, 36, each of which is associated with a respective disk surface and supported by an associated arm assembly. The heads 30, 32, 34, 36 are attached to a common accessing means or actuator, such as a VCM 40. Thus the heads 30, 32, 34, 36 are all maintained in a fixed relationship with one another relative to the radial position on their respective disk surfaces.
The output of the dedicated servo head 32 is supplied to an amplifier 42 and then to a demodulator 44. Demodulator 44 processes the servo information signal from the disk surface 22 and demodulates it to generate an analog PES. The PES from the demodulator 44 is an indication of the position of the servo head 32 away from the nearest servo track centerline, and thus the position of the data heads 30, 34, 36 away from the centerlines of the nearest data tracks on the respective disk surfaces 20, 24, 26.
A microprocessor 50 is connected to a random access memory (RAM) 52 and a programmable read only memory (PROM) 53 via a data bus 54. The disk file control unit 56 is also connected to data bus 54. A control unit 56 provides numerous commands to the microprocessor 50, including a signal, t.sub.d, representing the target track, and a signal, RZ, representing a "re-zero" to initialize the servo control system. Not shown in FIG. 1 are address and control lines for the microprocessor 50. The analog portion of the servo control system is shown essentially to the right of the data bus 54 in FIG. 2.
The signal read by the servo head 32 is input to the amplifier 42 and then the demodulator 44. While the control circuit is operable with any of numerous types of servo patterns and servo signal demodulation techniques, the servo control system will be explained with reference to a quadrature servo pattern.
The quadrature pattern on the servo surface 22 is demodulated by the demodulator 44 in the following manner. First, the demodulator 44 receives the quadrature servo signal from the amplifier 42 and generates two separate analog waveforms, which are designated primary (PESP) and quadrature (PESQ). The analog PESP and PESQ signals from the demodulator 44 are sent to analog-to-digital (A/D) converters 58, 59, respectively, which are constructed according to the present invention. The discrete values of PESP and PESQ at any sample time are designated PESP(n) and PESQ(n), where n represents a time index for each digital sample. The digital signal samples PESP(n) and PESQ(n) are then used by the microprocessor 50 to determine on which of the four tracks in one of the four-track bands of the quadrature pattern the servo head 32 is located. Once that has been determined, then the selection of the correct signal, i.e. either PESP(n) or PESQ(n), is made to determine PES(n).
The determination of the track type, and PES(n), and further operation of a digital servo control system is described in the previously-cited Workman patent.
Referring again to FIG. 1, the demodulator 44 also provides directly to the data bus 54 a one-bit digital signal, GBOD, to indicate that servo head 32 is located over the "guard band outside diameter", i.e. the radially outermost head position in the disk file. This signal is generated from a special code recorded on the radially outermost track on servo surface 22.
An integrating power amplifier (IPA) 64 provides the analog control current, i(t), to the VCM 40 and as feedback to an A/D converter 60. The A/D converter 60 provides to the data bus 54 a digital current sample, i(n), corresponding to the sample of analog current, i(t).
Thus, as shown in FIG. 1, the data inputs to the microprocessor 50 of the digital servo control system for purposes of this explanation are control unit commands for target track t.sub.d and re-zero RZ, head position relative to nearest track centerline PES(n), which is determined from PESP(n) and PESQ(n), VCM control current i(n), and GBOD.
A digital control signal, u(n), is output by the microprocessor 50 to a digital-to-analog converter (DAC) 62. As shown in FIG. 1, A/D converters 58, 59, 60 are driven by the same clock input so that the digital sampling of PESP, PESQ and i, all occur simultaneously. The output of the control signal u(n) computed from the digital sample of PESP(n) and i(n) occurs after a fixed calculation delay time and before the input of the next digital sample of PESP(n+1), PESQ(n+1) and i(n+1). The DAC 62 provides the analog control signal, u(t), to the IPA 64.
The microprocessor 50 also sends a special inhibit signal to the IPA 64. The IPA 64 is inhibited when the servo control system is first turned on to prevent any initial signal conditions from causing undesired movement of the VCM 40.
Several techniques are available to convert the analog signal into a digital form in the A/D converters 58, 59, but most of these techniques require large input voltage swings, and long periods of time to produce an accurate digital signal.